# Re: DAB+/Wi-Fi radios due by end of the year

• From: "DAB sounds worse than FM" <dab.is@dead>
• Date: Fri, 31 Aug 2007 21:16:16 GMT

Boltar wrote:
On Aug 31, 9:06 pm, Richard Evans <R.P.Evans.NoS...@xxxxxxxxxxxx>
wrote:
MP3 performs a discrete cosine transformation, which could be done
using floating point, however it is also possible to avoid the use
of floating point by using a simple integer method. BTW. I say this
based on my experiences of creating jpeg decoders (jpeg also uses
DCT, and I assume that the techniques would be similar with mp3).

For those of us where cosine means the length of one side of a right
angled triangle depending on the angle , what exactly is a DCT and how
is it used?

It's used on MP3, AAC/AAC+, MPEG-2 video, MPEG-4 video, JPEG, and so on.

Its use in audio codecs consists of taking a vector of time-domain audio
samples then the DCT is taken (actually the modified DCT (MDCT)) of the
vector of samples to produce the frequency domain representation - the
transform is lossless, so it's just a different representation of the same
information.

To achieve compression codecs like AAC and MP3 simply encode the frequency
domain samples with fewer bits (or zero bits) than are needed for a lossless
representation - and they do that by taking advantage of frequency and time

You've heard of the FFT, right? The DCT is very similar to that apart from
the fact that the FFT of a signal produces complex (real + j*imaginary, I
don't mean complex as in "difficult") samples, whereas the DCT outputs
real-valued samples.

--

.

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• Re: DAB+/Wi-Fi radios due by end of the year
... floating point, however it is also possible to avoid the use of floating ... experiences of creating jpeg decoders (jpeg also uses DCT, ...